The present invention relates to a method and an installation for the utilization of residual heat in pulp mills, where spent cooking liquor is evaporated with vapour in a multi-stage evaporator.
In pulp mills with highly advanced recovery, including combustion of spent cooking liquor, and electric generation in a counter-pressure turbine, a considerable surplus of hot water is given at temperatures in the range of 50.degree.-60.degree. C.
It has been proposed to utilize this hot water in district heat distribution networks but, on one hand, a condition therefor is a certain geographical location of the mill in relation to prospective consumers and, on the other hand, the piping always entails a heavy cost. Moreover, in district heating plants the temperature of available water is lower than what is normally the case. Finally, the demand for district heating is least in summer when the supply of surplus heat in the mill is at its highest.
A large heat consumer in the process is the evaporation plant for the above-mentioned liquor, which becomes an essential producer of 50.degree.-60.degree. C. water.
The desire for saving heat energy has since long resulted in arranging the evaporation in several successive stages or so-called effects. Usually five, but also four or six effects may be provided. At an initial temperature of about 140.degree. C. of the vapour discharged from the counter-pressure turbine, the temperature on the liquor side of the last effect will be in the range of 55.degree.-60.degree. C.
The lower limit is, for one thing, due to difficulties with pulsations which physically depend on the course of the vapour pressure curve and the heights of the liquid columns in the evaporation apparatuses. Thus, the drop in temperature between about 60.degree. C. and ambient temperature cannot be used for evaporation. The hot water produced during the condensation of the outlet vapour contributes to the low-temperature surplus in the mill.
A steam turbine is actually the only machine element that can utilize the drop in heat between 60.degree. C. and ambient temperature in a rational manner. To allow the vapour after the evaporation plant to pass a vacuum turbine for final expansion is theoretically possible and also right but it meets with certain practical difficulties. It will be a matter of extremely heavy pipelines and particularly expensive turbines and to this should be added the risk of deposits on the turbine blades from pulp particles coming from the liquor and entrained with the vapour, and problems with the compression of uncondensable gases introduced with the liquor.